In the manufacture of refiner-mechanical or thermomechanical fiber pulps from lignocellulose containing material, the material treated in the grinding apparatus normally is supplied with so much of energy that moisture accompanying the material is rapidly vaporized due to temperature above 100.degree. C. produced by friction. In order to prevent the material from being dried out during the grinding procedure with consequent exposure to excessive temperatures which are undesirable for the process, a quantity of water is added during the grinding operation to replace the amount of water evaporated from the material.
This results in the generation during the grinding procedure which steam must be removed from the grinding apparatus so as to avoid interference with the flow of material through the apparatus. In order to make full use of the heat content of the steam, the steam removal must be effected under the highest possible pressure without use of arrangements which might interfere with the pulping process.